218 FRESXEL. 



on the same subject, I find myself still carried back, by 

 the nature of Fresnel's labours, to the subject of double 

 retraction : but, this time, instead of occupying myself 

 with the manner in which the rays divide in passing 

 through certain crystals, I will examine the permanent 

 modifications which they receive : I will present, in one 

 word, the principal features of the new branch of optics 

 which bears the name of polarization of light. 



Every ray of light foiling even perpendicularly on any 

 surface, natural or artificial, of the transparent crystals of 

 carbonate of lime, called also calc spar, or Iceland spar, 

 is divided into two. One portion passes through the 

 crystal without deviation, which we call the ordinary ray; 

 the other undergoes a sensible retraction, and for that 

 reason has very justly the name of the extraordinary ray. 

 Both the ordinary and extraordinary ray lie in one plane 

 perpendicular to the face of the crystal. The considera- 

 tion of this plane is important, for it is this which deter- 

 mines the direct ion which the extraordinary ray will 

 take ; and in consequence a special name has been given 

 to it, "the principal section." 



These points being premised, I will suppose, to fix the 

 ideas, that a particular crystal of calc spar has its princi- 

 pal section directed north and south. P>elow this, and at 

 any distance, we will place another similar crystal turned 

 similarly ; that is, so that its principal section shall also 

 lie in the meridian. What will result from this disposi- 

 tion, if light traverse the whole system ? A single ray 

 impinges on the first crystal but it emerges in two rays : 

 each of these again seems as if it should undergo a double 

 refraction in the second crystal ; and thence we might 

 expect four emergent rays. Yet this does not happen. 

 The rays emanating from the first crystal are not divided 



